1. Field of the Invention
The present invention relates to a power steering system wherein power assistance derived therefrom will be modulated according to a vehicle operating condition such as vehicle speed.
2. Description of the Prior Art
In maneuvering a steering wheel of an automotive vehicle, it is recognized that power assistance at high vehicle speeds may desirably be less than at low speeds to allow for stabilization of the steering wheel at high speeds.
A power steering system with the above-mentioned steering characteristics is known having a reaction device which in accordance with fluid pressure supplied thereto provides resistance against relative rotation between valve elements of a servo-valve. The servo-valve controls fluid flow to and from a fluid motor according to the relative rotation between the valve elements, which, in turn, generates the power assistance. The fluid pressure supplied to the reaction device is controlled by a solenoid-operated control valve at a low level when high power assistance is necessary at low vehicle speeds, and on the contrary, at a high level when less power assistance is required at high vehicle speeds.
Conventionally, a flow dividing valve is provided to divide pressure fluid from a pump in accordance with a predetermined ratio into a first fluid flow directed to the servo-valve and a second fluid flow directed to the reaction device.
In the event, however, where a spool valve of the flow dividing valve is locked due to the existence of a foreign particle, for example, between the spool and cylinder of the flow dividing valve, so as to block the outlet port of the flow dividing valve which is connected to the servo-valve, the entire pressure fluid from the pump is directed to the reaction device. As a result, the pressure of the fluid derived from the pump rises to the threshhold value of pressure of a relief valve connected to the pump even when the servo-valve is sustained at a neutral position because the excess fluid flow is throttled by an orifice of the flow dividing valve and the solenoid-operated control valve. Accordingly, there is concern that seizure occurs in the pump so as to cause a breakdown of the power steering system.